This invention relates to a device for producing prestressed concrete members, such as concrete cross ties, concrete piles, concrete boards, according to the post-tensioning method.
Hitherto, for introducing a prestress into a precast concrete member according to the post-tensioning method, the end of a tensioning steel member is fastened to an anchor plate placed adjacent to the end surface of a concrete member to be formed. However, this method suffers from disadvantages in that the edges or ends of an anchor plate and tensioning steel member are exposed from the surface of a concrete member, so that there are experienced some difficulties in stopping the ingress of water through the joint of one concrete member to another, in addition, this concrete member involves an appearance problem.
Furthermore, when a tensioning member is fastened to an anchor plate with nuts at its opposite ends, a tension applied to the tensioning member is received by the male and female screw threads of tensioning steel and nuts, respectively, which are in meshing relation, and, as a result, there is created gaps between the screw threads, so that there is a danger of embrittlement cracking in the abutting or meshing portions of the both male and female threads, or there results in loosened engagement of a tensioning steel member with nuts, thus lowering the tension being applied thereto.
In addition, according to the conventional post-tensioning method, antifriction agent or sliding material such as water glass or asphalt is applied to the mid portion of a tensioning steel wire or bar, and the steel wire or bar is buried in a concrete so that the tensioning member is isolated from the concrete surrounding same, thus preventing local concentration of a prestress, when the prestress is introduced to a concrete member. However, this still fails to meet the success, because water glass or asphalt is apt to adhere to hands, molds or other members when placing steel wires or bars in a mold, so that the concrete is peeled from such members due to the adhesion of the sliding material or antifriction material. In addition, water glass or asphalt may possibly be included into concrete. For avoiding this shortcoming, vinyl tape or the like is wound around a steel wire or bar, to which has been applied water glass or asphalt. This apparently results in increase in expense and man hours and thus is not economical.